A conductive roll used as a charging roll of an electrophotographic copying machine has been required to have conductivity within the electric resistance range from about 10.sup.3 .OMEGA. to about 10.sup.9 .OMEGA. (measured using an electrode having an area of 1 cm.sup.2), and usually comprises a metal shaft and a conductive layer formed on a peripheral surface thereof.
In order to fully function as the charging roll, it has been considered that such a conductive roll is desirable to have an electric resistance level ranging from about 10.sup.3 .OMEGA. to about 10.sup.9 .OMEGA. as described above. The charging roll is pressed on a peripheral surface of a photosensitive drum for rotation to charge the peripheral surface of the photosensitive drum by sliding friction of the contact portion. The charging roll is therefore required to have elasticity. Accordingly, the above-described conductive layer of the charging roll is generally formed of a conductive rubber composition in which a conductive powder (carbon black, a metal powder or the like) or conductive fibers (carbon fibers or the like) are incorporated in synthetic rubber such as silicone rubber. Further, a charging roll has also been known which is provided with an ionic conductive elastic layer utilizing the inherent ionic conductivity of the synthetic rubber or enhanced in the ionic conductivity of the above-described synthetic rubber by addition of a high dielectric liquid or an ionic substance thereto. However, repetition of charging using the charging roll having these conductive rubber layers, with the conductive rubber layer being in direct contact with the surface of the photoreceptor, introduces the problem that low molecular weight components contained in the conductive rubber layer are transferred to the photoreceptor to cause image defects. Further, the rubber is worn away by contact with the photoreceptor to largely change the unevenness of the surface of the roll from the initial state, thereby impairing the uniformity of charging. Furthermore, high conductivity of the conductive rubber layer produces so-called pinhole leak, the phenomenon that excess current flows in defective portions of the photoreceptor, resulting in appearance of image defects.
In order to solve such problems, the surface of the conductive rubber layer is provided with resins low in electric resistance as protective layers. In this case, however, there is the problem that the electric resistance varies depending on the environmental conditions, resulting in changes in image density. For this reason, attempts have been made to add a conductive material such as carbon black to the surface resin layer (hereinafter sometimes referred to outermost layer). It is however impossible to prevent the dielectric breakdown of the photoreceptor or the charging roll in case of certain kinds of defects. It has been therefore tried to add conductive particles having an electric resistance of 10.sup.1 to 10.sup.5 .OMEGA..multidot.cm to the resins [JP-A-64-66675 (the term "JP-A" as used herein means an "unexamined published Japanese patent application")]. In this case, however, charging ability is insufficient in high speed charging in which the process speed of charging exceeds 100 mm/second, and particularly, it is difficult to maintain stable charging characteristics in long-term use.
As described above, all of the conventional charging rolls suffer from the problems of photoreceptor contamination, pinhole leak, changes in electric resistance depending on the environmental conditions, poor charging in high speed charging and deterioration of maintaining property of charging characteristics in long-term use, and therefore, no satisfactory roll has been obtained.